Liquid crystal devices incorporating a ferroelectric smectic liquid crystal material (FLCDs) are particularly suitable for use in displays and shutters in which their fast switching times and memory characteristics are of advantage. A conventional FLCD cell comprises a layer of ferroelectric smectic liquid crystal material contained between two parallel glass substrates, electrode structures being typically provided on the inside facing surfaces of the glass substrates in the form of row and column electrode tracks which cross one another to form an addressable matrix array. Prior to assembly of the substrates and filling of the cell with liquid crystal material, the inner surface of one or both substrates is treated to impart a preferred surface alignment direction and preferably a surface pretilt to the contacting molecules of the liquid crystal material layer.
The switching behavior of the liquid crystal molecules is dependent on the arrangement of the molecules in microlayers which, in the case of chiral smectic material, extend transversely of the substrates and adopt a chevron geometry having two possible states, C1 and C2, ad disclosed in J Kambe et al, Ferroelectric (1991), vol. 114, pp 3. Both C1 and C2 states can form as the material cools down from an isotropic phase to the chiral smectic phase during manufacture, and the boundaries between these two states may be seen as a zigzag defect. When used in a display device, material incorporating both the C1 and the C2 states can appear patchy, and it is therefore preferred that the material should be in one state for a practical device. The C2 state is preferred as it allows faster switching at lower voltages.
FLCD's are typically produced by assembling together the two glass substrates after they have been provided with the required electrode structures and appropriate surface treatment, typically involving spinning on a thin polymer alignment layer which is then rubbed to impart a preferred surface alignment direction, spacers being provided to space the two substrates apart by a small amount, typically in the range of 0.5 .mu.m to 50 .mu.m. The cell may then be filled by placing it above a bath of liquid crystal material, which is heated to a temperature at which it is in the isotropic phase, so that the bottom edges of the substrates are in contact with the material within the bath, and by then applying a vacuum so as to slowly draw the liquid crystal material upwardly between the substrates by capillary action. Alternatively the cell may be filled by injection of the liquid crystal material between the substrates. After filling of the cell, which may take a number of hours, the heat is removed and the cell is cooled down very slowly so that the liquid crystal material passes from the isotropic phase through the cholesteric and smectic A phases to the chiral smectic phase (usually the chiral smectic C phase) as the material cools (one or more of these phases may be omitted in certain materials). It is known to impart a pretilt by means of the alignment layer which favours the C2 state on cooling of the material. High values of pretilt tend to favour the C1 state, whereas low or medium values of pretilt tend to favour the C2 state. The relevant criteria are described in more detail in J C Jones, M J Towler, J R Hughes. "Fast, high-contrast ferroelectric liquid crystal displays and the role of dielectric biaxially". Displays (1993). vol. 14, no. 2, pp 86. However it can be difficult to obtain large areas of the C2 state in certain materials.